英语邀请函的主题句通常用于明确表达邀请的意图和内容，以下是一些常见且合适的表达方式：

I am writing to invite you to…

I think it would be a great idea if you could participate in…

I wonder if you can come…

We are pleased to invite you to attend…

It is our pleasure to invite you to attend…

We are longing to see you soon.

We sincerely hope you can attend.

I am delighted to extend an invitation to you for the upcoming…

It is with great pleasure that I am writing to invite you to…

这些句子可以根据具体场合和需要进行选择和调整，以确保邀请函既礼貌又具有个性化。